Expose defaultValue()

Summary:
SettingsMetadata provides a type-erased view, so the default value is stored as a string representation.
This method allows getting the actual original default value.

Differential Revision: D10077879

fbshipit-source-id: 2c6bf15eba57da5cb33ad1c657bc9bd51bcd2946

folly/experimental/settings/Settings.h

@@ -63,6 +63,16 @@ class SettingWrapper {
     core_.set(t, reason);
   }
 
+  /**
+   * Returns the default value this setting was constructed with.
+   * NOTE: SettingsMetadata is type-agnostic, so it only stores the string
+   * representation of the default value.  This method returns the
+   * actual value that was passed on construction.
+   */
+  const T& defaultValue() const {
+    return core_.defaultValue();
+  }
+
   explicit SettingWrapper(SettingCore<T>& core) : core_(core) {}
 
  private:

folly/experimental/settings/detail/SettingsImpl.h

@@ -327,6 +327,10 @@ class SettingCore : public SettingCoreBase {
     *settingVersion_ = nextGlobalVersion();
   }
 
+  const T& defaultValue() const {
+    return defaultValue_;
+  }
+
   SettingCore(
       SettingMetadata meta,
       T defaultValue,

folly/experimental/settings/test/SettingsTest.cpp

@@ -58,6 +58,10 @@ struct UserDefinedType {
     }
   }
 
+  bool operator==(const UserDefinedType& other) const {
+    return value_ == other.value_;
+  }
+
   int value_;
 };
 /* Note: conversion intentionally to different strings to test that this
@@ -203,6 +207,17 @@ TEST(Settings, basic) {
     EXPECT_FALSE(
         sn.setFromString("follytest_nonexisting", "300", "from_string"));
   }
+  EXPECT_EQ(
+      some_ns::FOLLY_SETTING(follytest, multi_token_type).defaultValue(), 123);
+  EXPECT_EQ(
+      a_ns::FOLLY_SETTING(follytest, public_flag_to_a).defaultValue(), 456);
+  EXPECT_EQ(
+      b_ns::FOLLY_SETTING(follytest, public_flag_to_b).defaultValue(), "basdf");
+  EXPECT_EQ(
+      some_ns::FOLLY_SETTING(follytest, some_flag).defaultValue(), "default");
+  EXPECT_EQ(
+      some_ns::FOLLY_SETTING(follytest, user_defined).defaultValue(),
+      some_ns::UserDefinedType("b"));
   {
     std::string allFlags;
     folly::settings::Snapshot sn;